Office chairs are widely used throughout the world. In general, such chair is equipped with a padded backrest and even a pair of armrests for providing a degree of comfort to the seated person. Further, some backrests in such chairs are height adjustable as detailed below.
A conventional height adjustable backrest is disclosed in, for example, U.S. Pat. No. 4,639,039 entitled "Height Adjustable Mechanism For Chair Backrest". Such mechanism 12 comprises a guide 24 secured to support 18 and a channel 28 secured to backrest 22 and mounted for vertical sliding movement on guide 24. In operation, bias latch pin 36 of latch bar 32 along slot 38 for engaging latch pin 36 with one of notches 51 through 56 so as to lock backrest 22 in one of height positions H1 through H6.
But this is unsatisfactory for the purpose for which the invention is concerned for the following reasons:
1. The positioning of torsion spring 40 is not well maintained. As such, torsion spring 40 is prone to disengage from the predetermined position thus failing the mechanism 12. PA1 2. The biasing of latch bar 32 is made possible totally by virtue of the spring force of legs 64, 66 of torsion spring 40. This may cause fatigue to torsion spring 40 after a short period of time of intensive use, resulting in a malfunction of torsion spring 40. PA1 3. It is possible that latch bar 32 is over headed in operation when torsion spring 40 is malfunctioned.
In view of the foregoing, it is desirable to provide an improved telescopic positioning mechanism for chair backrest in order to overcome the above drawbacks of prior art.